The Taoiseach and Tánaiste are open to the idea of a criminal investigation into the whistleblower smear campaign and also to the possibility of an outside police force doing the job.

However, they said Mr Justice Peter Charleton’s commission of investigation should get under way first and only if evidence of criminal behaviour emerges either during or at the conclusion of those proceedings should the question of a parallel or follow-on investigation be decided.

Sgt Maurice McCabe has said his preference would be for an external criminal probe and he is backed in that call by Labour leader Brendan Howlin who urged a “fresh approach” rather than another commission — the third one to probe the whistleblower affair.

Taoiseach Enda Kenny said he would not rule some form of criminal investigation but said that Judge Charleton was “ready to go” and should be allowed get his probe under way.

“In order to have a criminal investigation you have to have evidence of criminality and that may well happen,” he said. “As is the case in any commission of investigation, when it’s completed or during the course of its work under way, there may be High Court injunctions or there may well be criminal investigations to follow that so I wouldn’t rule it out and the terms of reference actually allow for that.”

Justice Minister Frances Fitzgerald also left the way open for a police investigation although she too said she wants to see what the Charleton Commission threw up first.

Commissions can not gather evidence for use in criminal investigations so if the behind-closed-doors proceedings hear that laws were broken, any subsequent Garda probe would have to begin a fresh trawl of information.

The use of external police to probe the gardaí would be a dramatic step requiring legislation. The FBI was called in here in 2000 to examine Garda actions during the Abbeylara siege which ended with the shooting dead of John Carthy but that investigation was headed by a senior Garda officer and the visiting officers were here to assist rather than lead the inquiry.

Gsoc, the Garda Ombudsman, has full powers to investigate suspected criminal behaviour by members of the force and prepare files for the DPP with recommendations on charges but it is already investigating two issues relating to the whistleblower affair and its resources are under strain.